Category page does not show any products

After playing around with the free version of HikaShop, we have now purchased the essential edition.
After configuring some categories
(3 levels deep in total:
root = product categories
1st level = Fly rods
2nd level = Scierra Salis fly rods

and creating one product:
Scierra Salis Fly Rod 9ft #9 4sec

I cannot get this product to show when clicking the 2nd category on the left hand module!
This was not an issue in my first attempt using the free HikaShop version, but now it is.
Not a clue what setting I am missing here which I have not tried yet....

Enabling the module 'random products' does show the one and only product but whatever I try, I cannot get it to show in the 2nd level category 'Scierra Salis fly rods'.

Anyone any idea's? (this should be a simple task but I am struggeling and have tried a lot already....)

Additional info:
- Using the standard protostar template
- Using the module type 'HikaShop Content Module' to show the category on the left hand side (position 7 in the Protostar template);
(I strongly suspect some incorrect settings in this particular module but am unable to find out what is causing this)

Solved it using your info but forgot to give some reply to that.
Many thanks